Hot Plug

The macro MODULE_DEVICE_TABLE automatically creates two variables. For the example: MODULE_DEVICE_TABLE (usb, usblp_ids); the variables __module_usb_device_size and __module_usb_device_table are created and placed into the read-only data section and the initialized data section of the module, respectively. The variable __module_usb_device_size contains the value of the size of the struct usb_id structure, and __module_usb_device_table points to the usblp_ids structure. The usblp_ids variable is an array of usb_id structures with a terminating NULL structure at the end of the list.

When the depmod program is run, as part of the kernel installation process, it goes through every module looking for the symbol __module_usb_device_size to be present in the compiled module. If it finds it, it copies the data pointed to by the __module_usb_device_table symbol into a structure, extracts all of the information and writes it out to the modules.usbmap file, which is located in the module root directory. It does the same thing while looking for the __module_pci_device_size in creating the modules.pcimap file.

With the kernel module information exported to the files modules.usbmap and modules.pcimap, our version of /sbin/hotplug can look like Listing 2 [available at ftp.linuxjournal.com/pub/lj/listings/issue96/5604.tgz]. This example only tests for a match of the USB product ID and vendor IDs. The Linux-Hotplug Project has created a set of scripts that covers all of the different subsystems that can call /sbin/hotplug. This enables drivers to be loaded automatically when new devices are inserted into the systems. It also starts up network services when network devices are seen. These scripts are released under the GPL and are available at linux-hotplug.sourceforge.net. Almost all major Linux distributions are currently shipping this package, so it is probably already on your machine.

The current /sbin/hotplug subsystem needs to be incorporated into other kernel systems, as they develop hot-plug capability. SCSI, IDE and other systems all have hot-plug patches available for kernel support but need to have script support, kernel macro support and modutils depmod support added in order to provide the user with a consistent experience.

As the kernel boots, and discovers new devices, it tries to spawn /sbin/hotplug, but since user space has not been initialized yet, it cannot run. This means that any USB or PCI devices that are needed at boot time need to be compiled into the kernel or exist in an initrd RAM disk image as a module. Sometime during the 2.5 development process, the initrd RAM disk image will be converted to contain an entire small user-space tree. This will allow /sbin/hotplug to be run during the boot process and load modules dynamically. Some links describing this disk image idea are: lwn.net/2001/0712/kernel.php3 -- marc.theaimsgroup.com/?l=acpi4linux&m=99705696732868 -- marc.theaimsgroup.com/?l=linux-kernel&m=99436439232254 and marc.theaimsgroup.com/?l=linux-kernel&m=99436253707952.

Because of the small space requirements of this RAM disk image, the dietHotplug program has been written. It is an implementation of the Linux-Hotplug bash scripts in C and does not require modules.*map files when the program runs. The executable size of the entire dietHotplug program is one-fifth of the size of the original modules.*map files themselves. The small size is due to the use of dietLibc (found at www.fefe.de/dietlibc) and other space-saving techniques. dietHotplug will undergo more development as the 2.5 kernel requirements are more fully known. dietHotplug can be downloaded from the Linux-Hotplug site.

I would like to thank David Brownell who wrote the original /sbin/hotplug kernel patch and most of the Linux Hotplug scripts. Without his persistence, Linux would not have this user-friendly feature. I also would like to acknowledge the entire Linux USB development team, who have provided a solid kernel subsystem in a relatively short amount of time.

Keith Owens wrote the supporting code in the depmod utility and has endured constant changes to the format of the MODULE_DEVICE_TABLE() USB structure.

The other developers on the linux-hotplug-devel mailing list who have helped with their patches and feedback on the hot-plug scripts also deserve recognition, along with the wonderful Linux distribution-specific support that Debian, Red Hat and Mandrake have provided.

This article was based upon a paper and presentation that I gave at the 2001 Ottawa Linux Symposium.